1. Field of the Invention
This invention relates generally to quadrature modulators, and, more specifically, to reducing or eliminating the unwanted sideband in the output of a transmitter comprising a quadrature modulator followed by a translational loop.
2. Related Applications
This application is related to U.S. patent application Ser. No. 09/515,633, entitled xe2x80x9cSYSTEM OF AND METHOD FOR REDUCING OR ELIMINATING THE UNWANTED SIDEBAND IN A SIGNAL DERIVED FROM THE OUTPUT OF A QUADRATURE MODULATOR,xe2x80x9d and U.S. patent application Ser. No. 09/515,538, entitled xe2x80x9cSYSTEM OF AND METHOD FOR COMPENSATING A BASEBAND SIGNAL TO REDUCE THIRD ORDER MODULATION DISTORTION,xe2x80x9d both of which are filed on even date herewith, both of which are owned in common by the assignee hereof, and both of which are hereby fully incorporated by reference herein as though set forth in full.
In a quadrature modulator, identified with numeral 30 in FIG. 1A, a complex baseband signal, i.e., a baseband signal having I and Q components, BBI and BBQ, assumed to be in quadrature (out of phase by 90xc2x0), is mixed with a complex local oscillator signal, also having I and Q components, LOI1 and LOQ1, and also assumed to be in quadrature, to form an output signal. The output signal is typically at an intermediate frequency and may be upconverted to the desired RF transmit frequency through various means such translational loop 120, which upconverts the output of the quadrature modulator by the frequency of a second local oscillator signal, LO2. The frequency of the signal is thus placed at the desired transmit frequency TX, where TX=LO1+LO2.
In the quadrature modulator 30, the I component of the baseband signal, BBI, is mixed, through mixer 31, with the I component of the local oscillator signal, LOI, and the Q component of the baseband signal, BBQ, is mixed, through mixer 32, with the Q component of the local oscillator signal, LOQ. The outputs of mixers 31 and 32 are then combined, through combiner 33, to form the output signal of the quadrature modulator.
If the I and Q components of the baseband signal, and the I and Q components of the local oscillator signal, are perfectly in quadrature, i.e., out of phase by 90xc2x0 exactly, and the components of the quadrature modulator are perfectly accurate, only the xe2x80x9cwantedxe2x80x9d sideband will appear at the output of the quadrature modulator, and no energy will appear in the xe2x80x9cunwantedxe2x80x9d sideband. This follows mathematically as follows: assuming BBI can be represented as A cos (xcfx89BBt+xcfx80/2), BBQ as A cos (xcfx89BBt), LOI1 as B cos (xcfx89LO1t+xcfx80/2), and LOQ as B cos (xcfx89LO1t), then the output of the quadrature modulator, (BBIxc3x97LOI1)+(BBQxc3x97LOQ1), reduces, through known mathematical identities, to Axc3x97B cos ([xcfx89BB+xcfx89LO1]t). This is the xe2x80x9cwantedxe2x80x9d sideband. As can be seen, there is no component at the xe2x80x9cunwantedxe2x80x9d sideband, i.e., at the frequency xcfx89LO1xe2x88x92xcfx89BB.
However, in the real world, there will be some inaccuracy in the quadrature of the baseband or local oscillator signals or in the components of the quadrature modulator. That will result in some energy at the unwanted sideband frequency. This situation is illustrated in FIG. 2B, which is a frequency domain representation of the output of the quadrature modulator in the case in which there is some inaccuracy in the quadrature of the baseband or local oscillator signals or in the components of the quadrature modulator. Representing baseband as a single frequency, the wanted sideband is identified with numeral 50, while the unwanted sideband is identified with numeral 51. (Representing baseband as a range of frequencies, the wanted sideband is identified with numeral 52, while the unwanted sideband is identified with numeral 53).
Typically, as illustrated, the amplitude of the unwanted sideband is less than that of the wanted sideband. However, in extreme cases, when the quadrature inaccuracy is large, the amplitude of the unwanted sideband can approach that of the wanted sideband.
In the case in which the transmitted signal is a phase-modulated signal, the presence of the unwanted sideband in the output of the quadrature modulator translates into phase error in the transmitted signal. This follows from the fact that the spectrum of the transmitted signal will simply be that of the signal output from the quadrature modulator, but translated upwards in frequency by the frequency of LO2. Thus, the frequency spectrum of the transmitted signal can be represented as shown in FIG. 2C. As can be seen, the spectrum is identical to that shown in FIG. 2B, except that all components thereof have been translated upwards by LO2.
Current GSM standards impose tight limits on the phase error of the transmitted signal. For example, under current GSM standards, the energy of the unwanted sideband should be less than that of the energy of the wanted sideband by 40 dB or more. Such limits are difficult if not impossible to meet with current quadrature modulators.
In xe2x80x9cAn ISM band Transceiver Chip for Digital Spread Spectrum Communicationxe2x80x9d, ESSCIRC 97, a circuit for generating the LOI and LOQ inputs to a quadrature modulator is described. The circuit is illustrated in FIG. 1B. The LOI and LOQ signals are provided by a divide by two circuit comprising two D-type flip-flops 2 and 3 driven by VCO 1. These signals are input to phase detector 4, which outputs a current proportional to any deviation from quadrature in the LOI and LOQ signals. This current is integrated by integrator 5 to produce an error voltage. The error voltage is input to comparator 6 along with the output from VCO 1. The error voltage is used to modify the mark-space ratio of the VCO output in order to correct for inaccuracies in the VCO and divide by two circuit.
There are several problems with this approach. First, it does not correct for intrinsic errors in the phase detector and comparator (see FIG. 1B).
Second, it does not correct for any inaccuracies in the components of the quadrature modulator (mixers 31 and 32, and combiner 33, in FIG. 1A).
Third, it does not correct for inaccuracies in the quadrature of the baseband signal.
Fourth, it requires a highly accurate phase detector in order to be effective.
Fifth, since it involves both detecting quadrature inaccuracy from and making corrections to the LO signal, a high frequency signal, it is difficult to achieve satisfactory results with this approach.
Accordingly, there is a need for a system for and method of reducing or eliminating the unwanted sideband in the output of a quadrature modulator followed by a translational loop which overcomes the disadvantages of the prior art.
In accordance with the purpose of the invention as broadly described herein, there is provided a system of and method for reducing or eliminating the unwanted sideband in the output of a transmitter comprising a quadrature modulator followed by a translational loop in which the presence of the unwanted sideband is detected through an unwanted sideband detector coupled to the translational loop. In one embodiment, the system comprises a baseband correction circuit, a quadrature modulator, a translational loop, and an unwanted sideband detector.
The I and Q components of the baseband signal, BBI and BBQ, are input to the baseband correction circuit. The outputs of the baseband correction circuit, BBIxe2x80x2 and BBQxe2x80x2, are input to the quadrature modulator as are the I and Q components of the local oscillator signal, LOI1 and LOQ1. The output of the quadrature modulator is input to the translational loop. The signal for transmission is derived from the output of the translational loop. The unwanted sideband detector is coupled to the translational loop. The output of the unwanted sideband detector is input to the baseband correction circuit.
In one implementation, the unwanted sideband detector is coupled to a low frequency signal generated within the translational loop. In one implementation example, the low frequency signal is taken from the input of a VCO within the translational loop.
Any inaccuracy in the quadrature of the baseband or local oscillator signals, or in the components of the quadrature modulator, results in an unwanted sideband in the output of the quadrature modulator and in the output of the translational loop. The unwanted sideband will also be reflected in the low frequency signal input to a VCO within the translational loop, and the implementation example referred to above exploits this property in detecting the presence and magnitude of the unwanted sideband from the signal input to the VCO.
In one embodiment, in a calibration mode of operation, a known baseband signal is applied to the baseband input of the quadrature modulator. The unwanted sideband detector detects the presence of the unwanted sideband, and provides a signal representative thereof to the baseband correction circuit. The baseband correction circuit iteratively revises one or more parameters responsive to the signal provided by the unwanted sideband detector and uses the one or more parameters to iteratively alter the I and Q components of the baseband signal. This process continues until the energy of the unwanted sideband is reduced to an acceptable level. At this point, the value of the one or more parameters is stored in a memory in the baseband correction circuit. Then, in a transmit mode of operation, after a real-world baseband signal is applied to the baseband input thereof, the baseband correction circuit alters one or more of the I and Q components of the real-world baseband signal responsive to the one or more parameters stored in the memory. The result is a corrected baseband signal, which is then modulated up to the transmit frequency by the quadrature modulator in combination with the translational loop. The signal for transmission is then derived from the output of the translational loop. In this mode of operation, the unwanted sideband detector may be deactivated or powered down since it is not used.
In one implementation, the relative phase relationship between BBI and BBQ is progressively adjusted in the calibration mode by the baseband correction circuit until the energy of the unwanted sideband is reduced to an acceptable level. In one implementation example, the baseband correction circuit progressively adjusts a variable relative delay xcfx86 between BBI and BBQ until the energy of the unwanted sideband is at the desired level. At this point, the value of xcfx86 is stored. Later on, during the transmit mode of operation, this value is retrieved and used to set the relative delay between the I and Q components of the baseband signal.
In one embodiment, a method of forming a transmit signal in accordance with the subject invention comprises the steps of retrieving one or more parameters; correcting one or more of the I and Q components, BBI and BBQ, of the baseband signal using the one or more parameters, thereby forming BBIxe2x80x2 and BBQxe2x80x2; quadrature modulating BBIxe2x80x2 and BBQxe2x80x2 respectively with the I and Q components of a local oscillator, LOI and LOQ, to form a modulated signal; and deriving the transmit signal from the modulated signal. In one implementation, the deriving step comprises upconverting the modulated signal to form the transmit signal.
In one embodiment, a method of calibrating a transmitter comprises correcting a baseband signal using one or more parameters; quadrature modulating the corrected baseband signal; detecting an unwanted sideband from a signal derived from the modulated signal; determining whether the unwanted sideband is below a desired threshold level; if so, storing the one or more parameters and ending the process; if not, revising the one or more parameters responsive to the detected unwanted sideband, and looping back to the correcting step, whereupon the foregoing process may be repeated one or more times. In one implementation, the foregoing process iterates one or more times until the unwanted sideband component is reduced to an acceptable level.
In one implementation example, the modulated signal is upconverted to a desired transmission frequency through a translational loop, and the detecting step comprises detecting the unwanted sideband from a low frequency signal generated within the translational loop. In one configuration, the low frequency signal is generated at the input of a VCO within the translational loop.
One advantage of the invention is the ability to correct for any inaccuracies in the components of the quadrature modulator.
Another advantage is the ability to correct for any inaccuracies in the quadrature of the I and Q components of the baseband signal.
A third advantage is that a highly accurate phase detector is not required.
A fourth advantage is that correction to a high frequency signal is not required. Instead, the invention only requires correction to the relatively low frequency baseband signal.
A fifth advantage is detection of unwanted sideband from a high frequency signal is not required. Instead, any unwanted sideband component is detected from a low frequency signal generated within the translation loop of the transmitter.
Additional advantages of the subject invention will be set forth in the description which follows, or will be apparent to one of skill in the art.